speeed racr,
Both turbos are good, it just depends on what you're after.
Those engine specs are real similar to the black 1994 Turbo we built, the "TX UFO" (
1994 Porsche Twin Turbo 3.8L | Marc, sorry about the broken thumbnails, we're working on it...)
That engine first did 560hp on an engine dyno as a 3.6TT with GT28R turbos. The turbos came on super fat but flat lined before 5,000rpm -- too small.
Then it got some larger custom-spec GT28R turbos and did over 700hp flywheel, eventually getting tuned to 645rwhp on 100-octane before it ran out of compressor flow. The midrange torque was the real killer, however -- from rolling starts, there were few cars on the road that could touch it. It ate up cars that ran 10.5 in the quarter.
It was later rebuilt as a 3.8L (got the very first set of Nickie 911 cylinders) and the subsequent owner wanted "more," even if it cost a little midrange torque. GT3071R turbos offered barely more top end, and robbed the midrange. GT30R turbos took it from 645rwhp to 752rwhp, but strong compressor surge was a problem despite having large external wastegates, 3" twin exhausts, and ported shroud compressor housings.
The best all-around for street? A set of our GT28R turbos. We used them on Avi's 3.8L 964 Biturbo, too -- engine dyno results here:
Dynos.jpg Check out the torque at 3,000rpm. That car is crazy quick.